\(\bullet \)  Is a higher concentration of H the key?
The indications are that a large fraction of H states at the Fermi surface, i.e.
reflected in either large \(N(0)\)  or a large ratio \(N_H/N(0)\) , is not a clear
determining factor, or at least is not essential.
This is a straightforward band structure quantity, and cannot be estimated
before the band calculation is done, because band structure effects cause
structure in \(N(\varepsilon )\) .
An example for this item: SH\(_3\)  is somewhat exceptional, with a strong van Hove peak
at the Fermi level.[1]} Pickard et al.[2]} and
Flores-Livas et al.[3]} have highlighted the
\(N(0)\)  factor in compressed hydrides.
